Polyethylene terephthalate (PET) is a well known resin for molding applications. Because of its partially crystalline structure, PET is particularly suitable for production of molded articles which are subjected to severe mechanical stress or exposed to severe heat. PET is frequently blended with various additives to improve properties such as impact strength, resistance to warping, flame resistance, thermal stability, etc. Mechanical properties of PET for molding purposes can generally be improved by the incorporation of reinforcing fibers such as glass fibers. The use of glass fibers for reinforcing PET molding composition is discussed for instance in U.S. Pat. No. 3,368,995.
PET crystallizes relatively slowly. This limits the utility of PET as a resin for the formation of articles by injection molding. Addition of nucleating agent such as talc reduces the crystallization time and allows PET to be molded more efficiently. Generally, however, very high mold temperatures are needed to produce acceptable parts in this manner. The use of mold temperatures in excess of about 130.degree. C. is not generally considered practical because most molds are heated with water and only reach temperatures of 85.degree.-110.degree. C. For this reason, molding temperatures below about 110.degree. C. are generally preferred. Fast molding times are considered desirable as an economic consideration. For use in making molded articles, the necessity for high mold temperatures and relatively long molding times is a disadvantage which is only partially compensated for by the exceptional rigidity and high heat distortion temperature of PET molding compositions.
Various attempts have been made to improve the molding properties of PET molding compositions. U.S. Pat. No. 4,086,212 and British Pat. No. 1,505,214, for instance, describe terephthalic copolyesters having a diol component which is mostly ethylene glycol, but which also contains codiols intended to modify the properties of the molding composition. The copolyesters of British Pat. No. 1,505,214 are said to crystallize considerably more rapidly than pure polyethylene terephthalate and to possess a very high melting point. Likewise, British published application No. 2,015,014 describes polyethylene terephthalate resin compositions containing selected low molecular weight organic esters of aromatic carboxylic acid, ketone, sulfone, sulfoxide, nitrile or amide for the purpose of improving the gloss of the molded product.
U.S. Pat. No. 3,565,852 describes PET molding compositions containing additional polyester of low polymer melt temperature such as polyneopentyl glycol terephthalate, polypentamethylene isophthalate, etc.
British Pat. No. 1,451,945, describes PET molding composition containing a variety of sulfonate salts including derivatives of alkali metal taurates and isethionates.